Electronic sensor devices manufactured by MEMS technology are playing key roles in many areas. For instance, micro mechanical gyroscopes have enabled several important control systems in transportation and commercial applications. Other microdevices such as pressure sensors, accelerometers, actuators, and resonators fabricated by MEMS technology are also used in many areas.
One type of micro gyroscope contains two movable proof masses. The proof masses are suspended above a substrate by a support structure. The proof masses are vibrated in the same plane (in-plane) at a predetermined frequency by a motor in the gyroscope. The motor may include electrodes that drive the proof masses in the same plane in an oscillatory manner. The oscillation of the proof masses is controlled to a frequency near the resonant frequency of the proof masses.
In addition to a set of proof masses and drive electrodes, the gyroscope also contains sensing electrodes around the proof masses that report signals indicative of the movement of each proof mass. In particular, certain electrodes sense the in-plane movement of the proof masses. Other electrodes sense the out-of-plane movement of the proof masses. With appropriate signal processing and extraction circuitry, an angular rate component can be recovered from the reported signal of the electrodes sensing the out-of-plane movement of the proof masses.
The proof masses and support structure in conventional gyroscopes are extremely thin. The thickness of beams in the support structure is known to be below 10 μm and the width of the beams below 5 μm, with very tight process tolerances. There is a need to make the proof masses thicker to improve yield in mass production and a need to relax process tolerances. This is particularly important in low cost gyroscope devices for automobile applications. Making the proof masses thicker, however, generates other problems. For instance, certain frequencies of various vibration modes become undesirable and may become more susceptive to signal noise.
A need exists for improved mechanisms to realign frequencies of different vibration modes of a gyroscope that uses movable proof masses. The mechanism should allow the sensor device to use thicker movable proof masses and support structures, which improves yield in mass production applications. It is, therefore, desirable to provide an improved mechanism in a sensor to overcome most, if not all, of the preceding problems.
While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.